1. Field of the Invention
This invention relates to the surface treatment of articles made from halide glasses and, in particular, to the surface treatment of fluoride glass articles. The surface treatment is carried out for the purpose of improving the surface quality.
2. Related Art
Halide glasses find particular application in the preparation of fibre, and the most usual way of preparing fibre is by stretching a suitable preform. It has long been recognised that the surface condition of the preform can have a substantial effect upon the performance of the fibre, and it is well established practice to treat the surfaces of halide glass articles in order to improve the performance of fibre which results therefrom.
The most commonly used articles in the preparation of fibres are as follows:
Preforms
A preform consists of a region formed of the core glass surrounded by a region formed of the cladding glass. The preform has the same configuration as the fibre but it is substantially shorter and substantially greater in cross-sectional area. The preforms are usually made by casting a tube of the cladding glass and, after the cladding has solidified but before it has substantially cooled, casting the core glass into the bore of the tube. At this stage the whole preform is allowed to cool to room temperature. This method gives a good interface between the core glass and the cladding glass, but the quality of the outer surface is often inadequate and the mechanical strength of the resulting fibre is substantially reduced. Standard preforms usually have a diameter of 10-20 mm.
Tubes
Tubes of halide glass are often utilised in the preparation of halide fibres. Tubes with 10-20 mm OD and 1-10 mm ID are particularly suitable. The tubes are used in conjunction with cast preforms as described above when it is desired to reduce the core/cladding ratio of the preform (i.e. to make the core smaller in comparison with the cladding).
This technique is used when it is desired to produce fibre with small cores. If the preform is made by casting, then it is either necessary to cast a small core into an inconveniently small bore or else it is necessary to cast an inconveniently large cladding to produce a preform which has an inconveniently large diameter for fibre drawing. These mechanical difficulties can be avoided by casting a preform with a convenient core size and stretching the preform so that its diameter is reduced by about 2-10 times. The diameter of the stretched preform is usually less than 10 mm and it contains too little cladding. The amount of cladding can be increased by the use of a tube of cladding glass.
Thus a cast and stretched preform can be placed in the bore of the tube and, using techniques known to persons skilled in the art, the tube can be shrunk until it engages with the outer surface of the preform and the outer surfaces become united, e.g. by fusion. In order for this process to operate satisfactorily it is necessary that the mating surfaces be clean and, therefore, it is necessary to provide treatments for cleaning the surfaces. Any defects which hinder the mating of the surfaces are liable to reduce the strength and increase the attenuation of the resulting fibre.
Rods
Rods cast from halide glasses are also used in the preparation of fibre and this provides an alternative to the use of cast preforms mentioned above. This technique comprises casting a rod of the core glass and then shrinking a tube of the cladding glass onto the rod using the technique mentioned above. When this technique is employed, the quality of the surface of the rod is of great importance because energy transfer usually extends into the innermost regions of the cladding which means that the core/cladding interface forms part of the path region. Therefore defects in the interface can have an unacceptable effect upon the attenuation of the fibre as well as having an adverse effect upon its mechanical properties. For this reason it is important that the surface of the rod and the inner surface of the tube have high quality. It should also be realised that defects in the outer surface of the tube may lower the tensile properties of the resulting fibre for the reasons given above.
Thus, it is well established that the surface quality of articles used in the preparation of halide glass fibres is of great importance and, therefore, surface treatments have been proposed. This treatment sometimes includes mechanical polishing which may use abrasives. Such techniques are valuable when casting leaves a rough surface. The polishing can remove the roughness. However polishing tends to leave contamination on the surface, for example the residues of the abrasives may remain on the surface. Even if mechanical polishing is not used the surface may be contaminated and such contamination may result in the defects enumerated above.
For this reason it is usual to clean the surface chemically, e.g. by removing a thin surface layer of glass so as to expose a clean and uncontaminated surface. It has been found that aqueous etchant solutions, e.g. solutions containing hydrochloric acid and zirconium oxychloride are particularly effective for cleaning the surface of a fluoride glass article. Etchants of this nature are described in U.S. Pat. No. 4,631,114.
Once the etching described above has been completed, it is necessary to remove the etchant from the surface of the glass article. This has been carried out by washing the article in pure water and thereafter removing the water by washing with organic solvents, which can be allowed to evaporate.
U.S. Pat. No. 4,898,777 uses an aqueous etchant based on H.sub.3 BO.sub.3 and this is removed by rinsing with water optionally containing HNO.sub.3 or HCl. A volatile solvent such as acetone or volatile alcohols (eg methanol, ethanol and isopropanol alcohol) may be used to assist drying.
We have found, most surprisingly, that the surface quality fluoride glass articles can be substantially increased by modifying the technique used to remove the etchant. This is particularly important where the glass article is a precursor for the preparation of optical fibre.